A fluid brake device conducts variable control of viscosity of magnetic viscosity fluid by causing a magnetic flux to pass through the magnetic viscosity fluid. The magnetic viscosity fluid is filled in a fluid chamber of a casing, and contacts a brake rotor. Braking torque is provided to the brake rotor of the fluid brake device with comparatively small electric power, so that the fluid brake device is suitably used in a variable valve timing apparatus that controls a relative phase between a crankshaft and a camshaft of an engine in accordance with the braking torque.
JP-A-2010-121614 (US 2010/0095920) describes a fluid brake device having a casing and a brake shaft penetrating the casing. A sealing structure is provided in a clearance between the brake shaft and the casing. Specifically, the sealing structure is constructed by a permanent magnet and a magnetic flux guide, which are arranged to surround the brake shaft along a rotation direction. Magnetic flux generated by the permanent magnet is guided from the magnetic flux guide to the brake shaft through a seal gap between the magnetic flux guide and the brake shaft communicating with a fluid chamber. As a result, the magnetic viscosity fluid flowing into the seal gap from the fluid chamber receives the magnetic flux, so that the viscosity of the magnetic viscosity fluid is raised. Thus, the magnetic viscosity fluid is caught in a film shape as a self-sealing film.
The self-sealing film formed in the seal gap restricts the magnetic viscosity fluid from flowing outward from the casing. Thus, the magnetic viscosity fluid is restricted from leaking from the fluid chamber, so that variation can be reduced in the braking characteristics. Further, the self-sealing film made of the magnetic viscosity fluid can reduce the friction resistance for the brake shaft. Therefore, when the fluid brake device is applied to the variable valve timing apparatus, torque loss can be reduced by the reduction in the friction resistance.
However, the magnetic flux generated by the permanent magnet may be guided to not only a part of the brake shaft opposing to the magnetic flux guide but also the other part not opposing to the magnetic flux guide. In this case, the density of the magnetic flux becomes small in the seal gap, compared with a predetermined value. If pressure-resistant properties of the self-sealing film are lowered by the reduction in the density of the magnetic flux, the magnetic viscosity fluid may leak from the casing.